1. Field of the Invention
The present invention relates to an area separating apparatus and an area separating method for judging whether or not each pixel belongs to a photograph image area or whether or not each pixel belongs to a thin line image area for use in an image processing apparatus for performing a so-called pseudo-halftoning process on image data. The invention further relates to an image processing apparatus employing such an area separating apparatus and method.
2. Description of Related Art
In image forming apparatuses such as digital copying machines, printers and facsimile machines, a so-called pseudo-halftoning process is performed when multi-level image data indicative of density gradation levels is converted into two-level image data. Particularly, multi-level image data representing a photograph image includes a lot of halftone image data, so that the pseudo-halftoning process is indispensable. One example of the pseudo-halftoning process is an error diffusion process.
In the error diffusion process, an error generated when multi-level image data of an object pixel is converted into two-level image data with the use of a predetermined threshold is diffused to peripheral pixels yet to be subjected to the two-level quantization around the object pixel for modifying multi-level image data of the peripheral pixels, and this error diffusing operation is repeatedly performed. Thus, the density of dots to be outputted from an image forming section is properly controlled, whereby an image represented on a pseudo-halftone basis is outputted. Image forming sections are mostly adapted to form an image on a recording sheet through an electrophotographic process.
In an ordinary error diffusion process, dot distribution is at random, resulting in generation of a lot of isolated dots. In the image formation through the electrophotographic process, however, it is impossible to accurately control the size of each of the isolated dots, because dots in an image do not always have a stable size. Where the ordinary error diffusion process is applied to the image formation through the electrophotographic process, gradation representation cannot satisfactorily be stabilized. Therefore, the ordinary error diffusion process is not always suitable for reproduction of a photograph image.
Another example of the halftoning process is an organized dithering process. The organized dithering process includes a dot distribution method (frequency modulation method) in which dots having the same size are distributed at various densities in accordance with gradation levels, and a dot concentration method (amplitude modulation method) in which dots having different diameters according to gradation levels are arranged with the centers thereof being equidistantly spaced from one another. Of these methods, the dot concentration method is more suitable for halftone representation through the electrophotographic process in consideration of the stabilization of the gradation representation.